For example, an optical recording medium such as a digital video disc and a blu-ray disc is such that in a reproduction optical system with a laser light of wavelength λ and a numerical aperture NA of an objective lens, the length of a reproducible recording mark is greater than or equal to the resolution limit (λ/4NA) with respect to a recording mark sequence in which the length of the recording mark is equal to the length of an adjacent non-recorded space.
As a method of reproducing a recording mark smaller than the resolution limit in such an optical recording medium, there has been investigated a technique for practically increasing the NA within a medium by adding, to the optical recording medium, a signal reproducing functional layer having a function to reduce the size of a laser light spot.
For example, there has been known a technique for obtaining super resolution reproduction at high carrier to noise ratio (CNR), which serves as an index of reproduction performance, with use of Ge2Sb2Te5 as a signal reproducing functional layer material, and with utilization of a variation in the refractive index due to liquid phase formation of this material (refer to Patent Document 1). Super resolution reproduction can be experimentally obtained in the case where the power of laser light irradiation for reproduction is increased.
Super resolution reproduction at similarly high CNR can be experimentally achieved also by use of a signal reproducing functional layer material of another composition containing Sb and Te, not limited to Ge2Sb2Te5. However, in any of these cases, there is a problem in that reproduction durability is insufficient for practical use when performing super resolution reproduction, as a result of the influence of temperature increase within the medium due to the increased power of laser light irradiation.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H5-258345    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H5-217211    Non Patent Document 1: Technical Digest of International Symposium on Optical Memory, 2000, p. 224-225.